BOS ROM for PMS B2P

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Pernod
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BOS ROM for PMS B2P

Postby Pernod » Wed Aug 09, 2017 6:11 pm

Anyone have the BOS ROM required to use the B2P from PMS? I believe Arcadian has/had one of these.
- Nigel

BBC Model B, ATPL Sidewise, Acorn Speech, 2xWatford Floppy Drives, AMX Mouse, Viglen case, etc.

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mlouka
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Re: BOS ROM for PMS B2P

Postby mlouka » Wed Dec 13, 2017 10:07 pm

Please find attached a zip file with a dump of the ROM and a SSD with the ROM on it (that can be run fra SW RAM).

Michael.
Attachments
PMSB2P_BOSROM.zip
(12.85 KiB) Downloaded 6 times
BBC Master 128, BBC Model B i7, Watford Electronics Solderless Sideways ROM board, PMS B2P-6502 2nd proc., PiTubeDirect (internal in Master, external for Beeb), RetroClinic Multi-OS Selector, Sundby 256k RAM/ROM card, TurboMMC, Cumana disc drives ....

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Pernod
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Re: BOS ROM for PMS B2P

Postby Pernod » Wed Dec 13, 2017 10:50 pm

Thanks, do you also have the system disc that came with it?
- Nigel

BBC Model B, ATPL Sidewise, Acorn Speech, 2xWatford Floppy Drives, AMX Mouse, Viglen case, etc.

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mlouka
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Re: BOS ROM for PMS B2P

Postby mlouka » Wed Dec 13, 2017 11:13 pm

Pernod wrote:Thanks, do you also have the system disc that came with it?


‘fraid not. Just the device, rom and manual.
BBC Master 128, BBC Model B i7, Watford Electronics Solderless Sideways ROM board, PMS B2P-6502 2nd proc., PiTubeDirect (internal in Master, external for Beeb), RetroClinic Multi-OS Selector, Sundby 256k RAM/ROM card, TurboMMC, Cumana disc drives ....

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mlouka
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Re: BOS ROM for PMS B2P

Postby mlouka » Thu Dec 14, 2017 10:35 am

mlouka wrote:
Pernod wrote:Thanks, do you also have the system disc that came with it?


‘fraid not. Just the device, rom and manual.


I just checked the manual and according to that, there was no system disc. "Chapter 1.1 Checklist of Items" states:

"As well as this User Guide, you should have the follwing items in the box you have just opened - the B2P Second Processor and the BOS ROM. Should either of these items are damaged or missing contact PMS without delay."

The only reference to a disk from B2P in the manual is regarding Hi-Wordwise Plus which B2P sold on disk and requires that Wordwise Plus is also installed on ROM. The manual mentions both "B2P Hi-Wordwise Plus" and "BBC Hi-Basic" as available via B2P. Given that Hi-BASIC for the Acorn processor works fine, I suspect that B2P Hi-Wordwise Plus is no different for the version for the Acorn 2nd processor from Computer Concepts but haven't tried.
BBC Master 128, BBC Model B i7, Watford Electronics Solderless Sideways ROM board, PMS B2P-6502 2nd proc., PiTubeDirect (internal in Master, external for Beeb), RetroClinic Multi-OS Selector, Sundby 256k RAM/ROM card, TurboMMC, Cumana disc drives ....

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Pernod
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Re: BOS ROM for PMS B2P

Postby Pernod » Thu Dec 14, 2017 11:43 am

mlouka wrote:I just checked the manual and according to that, there was no system disc.

Yeah, I saw that in the manual. I was referring to http://chrisacorns.computinghistory.org ... mDiscs.pdf, but it doesn't actually say what's on the discs.
I'm not sure how easy it is to get into the B2P but would you be able to get a good photos (so that all IC's are identifiable) of both sides of the board? Does anyone have any theories on how the 65C02 boots without a ROM present on the board? The last page of the user guide would've been much more useful if the author didn't run out of coffee!
- Nigel

BBC Model B, ATPL Sidewise, Acorn Speech, 2xWatford Floppy Drives, AMX Mouse, Viglen case, etc.

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BigEd
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Re: BOS ROM for PMS B2P

Postby BigEd » Thu Dec 14, 2017 11:57 am

Pernod wrote:Does anyone have any theories on how the 65C02 boots without a ROM present on the board?

You've got me interested now!

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Pernod
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Re: BOS ROM for PMS B2P

Postby Pernod » Thu Dec 14, 2017 12:19 pm

We know it uses the NPFD line from the 1MHz bus and so must map page &FD into the 65C02 address space, then trigger a reset. It transfers &800 bytes from the ROM into &F800-&FFFF. Need to take a closer look at the ROM to understand further.
- Nigel

BBC Model B, ATPL Sidewise, Acorn Speech, 2xWatford Floppy Drives, AMX Mouse, Viglen case, etc.

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jms2
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Re: BOS ROM for PMS B2P

Postby jms2 » Thu Dec 14, 2017 1:22 pm

I can explain the workings of the E2P, which is the Electron equivalent of the same thing. I also have a commented disassembly somewhere of the E2P code.

However, the B2P can't work in exactly the same way as the E2P. This is because the E2P plugs into a cartridge slot on the Plus 1 and is therefore occupying a rom slot by default, whereas the B2P plugs into the Tube and the 1MHz bus.

The E2P hardware is arranged so that a 256-byte window of Second Processor memory (&FF00-&FFFF) also appears at &8000-&80FF at the relevant ROM slot, in other words you have 0.25K of sideways RAM! When the software is loaded from disc it is able to copy 256 bytes to this location. This then causes three things to happen:

1) Writing to this area generates an interrupt (because of the E2P hardware) and this brings the second processor out of reset.
2) Because the data occupies &FF00-&FFFF, it provides all the vector addresses including (&FFFC). This jump runs a short routine which copies across the remainder of the OS into &F800-&FF00.
3) Subsequent presses of Break retain the second processor environment because some rom header code is retained, which means that when the sideways rom is polled it jumps straight back into the Tube code.

I think what must be happening is that the "window" appears in page &FD on the BBC micro rather than at &8000. I guess there are similar arrangements on the BBC to catch Break presses and do something with 1MHz bus devices?

There is a ROM version of the E2P software, but because the Electron has to maintain the E2P "fake rom" plugged in all the time (except during disc accesses), the code cannot actually run from there. What it does is copy itself into RAM (not only at &400, but also at about &2000 I think) and run from there. The BOS ROM is presumably different; whilst it copies the Tube code to &400 as usual, any * commands can remain accessible from the ROM.

The manual also mentions DRAM refresh. This is done on the E2P by means of a regular clock-generated NMI signal which accesses a a rotating block of memory on a periodic basis. This obviously places a load on the processor, dragging down its performance a bit more than the advertised 2Mhz clock speed would suggest.


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